Use CLOB for timezone

TEXT defaults to a length of 255 which is going to fail in some cases as the timezone can be rather long.

This changes it back to a CLOB as it has been before as well: https://github.com/owncloudarchive/calendar/commit/8d8bb68b010fc2c42a258dee43516404a95ab861. I'm not super convinced that CLOB is the best choice here but at least it seems to work.

Fixes https://github.com/owncloud/core/issues/22876